There are fill level measurement systems having fill level sensors that are immersed into a liquid are used in particular in motor vehicles. Detection of the liquid level is accomplished, for example, using capacitative fill level sensors, the capacitance of the sensor structures being influenced by the liquid level and by the dielectric properties of the liquid.
In determining a time-related fill level signal, however, the problem often occurs that the fill level fluctuates considerably, e.g. when the motor vehicle is being moved.
Therefore, a determination of the resulting fill level signal may be made over time in order to estimate the quantity of liquid present, e.g. the quantity of engine oil or fuel.
The fill level sensors are embodied as elongated sensor elements that are occupied by sensor structures along the fill level region that is to be detected. With highly viscous liquids in particular, the problem caused by a fluctuating liquid level may be that a liquid film forms on sensor surfaces that are briefly wetted. In the case of analog capacitative or thermal fill level sensors, for example, this liquid film may cause a positive error signal. The sensor signal for calculating the instantaneous fill level is then too great because of the liquid film left behind.
After a while, these liquid films run off again as a function of the liquid's properties, such as density and viscosity. As a result, the error signal disappears. With a fluctuating fill level, however, this liquid film is renewed at regular intervals, so that a fill level signal averaged over time exhibits a positive error.